DEPTH OF FLUID FLASHING AT THE ARISTA EPITHERMAL DEPOSIT, MEXICO

The Arista intermediate- sulfidation epithermal vein deposit in Oaxaca, Mexico, consists of two distinct vein sets containing precious and base metal ores over 1 km of known strike lengths and a known depth extent of 600 m. The ore in the veins displays typical banded and cockade textures with an increased frequency of brecciated and fragmental textures deeper in the orebody. Fluid inclusion microthermometry from selected fluid inclusion assemblages in representative vein samples indicate the hydrothermal fluid experienced phase separation by flashing episodically over the duration of the deposit formation. The textural characteristics of quartz and associated ore minerals indicate two different precipitation regimes. Episodes of relatively slow solute precipitation resulted in the formation of barren bands consisting dominantly of large euhedral quartz grains. Rapid solute precipitation is evidenced by textures that include dendritic ore minerals; stacked fluorite octahedra; acicular carbonate minerals; and recrystallized quartz forming mosaic, feathery, or dentine textures. These textures and the associated mineral assemblages formed under non-equilibrium conditions and can likely be explained by the occurrence of flashing. Based on fluid inclusion data, periodic flashing at Arista must have occurred to a depth of ~1 km below the paleosurface.